This invention relates to an addition reaction-curing electrically conductive silicone composition and to a method for the preparation of an electrically conductive cured silicone. More particularly, this invention relates to an addition reaction-curing electrically conductive silicone composition that cures to a silicone with the desirable properties of low electrical resistance and resistivity, electrical resistance independent of temperature, and little variation in electrical resistance and resistivity over time. The invention also relates to an efficient method for fabricating the electrically conductive cured silicone.
Electrically conductive silicone compositions that cure by addition reaction to give electrically conductive cured silicones are known (below, "electrically conductive" is abbreviated simply as "conductive"). Examples include the conductive silicone rubber compositions disclosed in JP-A-3-170581 and JP-A 7-133432. These compositions comprise organopolysiloxanes having at least 2 alkenyl groups per molecule, organopolysiloxanes having at least 2 silicon-bonded hydrogen atoms per molecule, silver micropowder, and platinum addition-reaction catalyst.
The conductive cured silicones afforded by these compositions suffer from problems such as high resistance and resistivity values, resistance dependent on temperature, and large variation in resistance and resistivity values over time. Although the cured products may have low resistance at room temperature immediately after cure, higher temperatures can cause the resistance to increase, and the passage of time can cause the resistance and resistivity values to rise.
One method for coping with these problems consists of increasing the amount of the metal micropowder in the composition, but this increases viscosity of the composition substantially, and hence, strongly impairs the handling characteristics. Another method for addressing the problems consists of increasing the addition of the metal micropowder to the composition while admixing a large amount of a volatile solvent to reduce viscosity and improve handling characteristics. However, the resulting composition, cured silicone, or both, may not be homogeneous, and the cured silicone will not inevitably have low resistance and resistivity values. Moreover, this method cannot give a cured silicone with small variation in resistance and resistivity over time.
However, cured silicone with the desirable properties can be obtained by blending a small amount of a special volatile solvent into an addition reaction-curing conductive silicone composition and effecting volumetric shrinkage of its cured product by removing the volatile solvent during or after curing. (The desirable properties are low resistance and resistivity values, resistance independent of temperature, and low variation by the resistance and resistivity values over time). Moreover, the desirable properties are obtained without having to increase the amount of metal micropowder in the composition and without having to blend large amounts of volatile solvent into the composition.
One object of this invention is to provide an addition reaction-curing conductive silicone composition that cures to give a conductive cured silicone with the desirable properties discussed above. An additional object of the invention is to provide a highly efficient method for fabricating the conductive cured silicone.